Download Object and Class in Java

Object and Class in Java
Object in Java
• An entity that has state and behavior is known
as an object e.g. chair, bike, marker, pen,
table, car etc. It can be physical or logical
(tengible and intengible). The example of
integible object is banking system.
An object has three characteristics:
• state: represents data (value) of an object.
• behavior: represents the behavior (functionality)
of an object such as deposit, withdraw etc.
• identity: Object identity is typically implemented
via a unique ID. The value of the ID is not visible
to the external user. But,it is used internally by
the JVM to identify each object uniquely.
• For Example: Pen is an object. Its name is
Reynolds, color is white etc. known as its state. It
is used to write, so writing is its behavior.
Definition
• Object is an instance of a class. Class is a
template or blueprint from which objects are
created. So object is the instance(result) of a
class.
Class in Java
• A class is a group of objects that has common properties. It
is a template or blueprint from which objects are created.
• A class in java can contain:
• data member
• method
• constructor
• block
• class and interface
Syntax to declare a class:
class <class_name>{
data member;
method;
}
Simple Example of Object and Class
class Student1{
int id;//data member (also instance variable)
String name;//data member(also instance variable)
public static void main(String args[]){
Student1 s1=new Student1(); //creating an object of Student
System.out.println(s1.id);
System.out.println(s1.name);
}
}
Output:0 null
Instance variable in Java
• A variable that is created inside the class but
outside the method, is known as instance
variable.Instance variable doesn't get memory
at compile time.It gets memory at runtime
when object(instance) is created.That is why, it
is known as instance variable
• Method in Java
In java, a method is like function i.e. used to expose behavior of an
object.
Advantage of Method
Code Reusability
Code Optimization
new keyword
• The new keyword is used to allocate memory
at runtime.
class Student2{
int rollno;
String name;
void insertRecord(int r, String n){ //method
rollno=r;
name=n;
}
void displayInformation(
){System.out.println(rollno+" "+name);}//method
public static void main(String args[]){
Student2 s1=new Student2();
Student2 s2=new Student2();
s1.insertRecord(111,"Karan");
s2.insertRecord(222,"Aryan");
s1.displayInformation();
s2.displayInformation();
}
}
111 Karan 222 Aryan
class Rectangle {
int length;
int width;
void insert(int l,int w){
length=l;
width=w;
}
void calculateArea() {System.out.println(length*width);}
public static void main(String args[]){
Rectangle r1=new Rectangle();
Rectangle r2=new Rectangle();
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}
Output:55 45
class Rectangle{
int length;
int width;
void insert(int l,int w){
length=l;
width=w;
}
void calculateArea(){System.out.println(length*width);}
public static void main(String args[]){
Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}
•
o/p : 55 45
Method Overloading in Java
• If a class have multiple methods by same
name but different parameters, it is known
as Method Overloading.
Different ways to overload the
method
• By changing number of arguments
• By changing the data type
• class Calculation{
• void sum(int a,int b){System.out.println(a+b);}
• void sum(int a,int b,int c){System.out.println(a+b+c);}
•
• public static void main(String args[]){
• Calculation obj=new Calculation();
• obj.sum(10,10,10);
• obj.sum(20,20);
•
• }
• }
• Test it NowOutput:30 40
Example of Method Overloading by
changing data type of argument
• class Calculation2{
• void sum(int a,int b){System.out.println(a+b);}
• void sum(double a,double b){System.out.println(a+b);}
•
• public static void main(String args[]){
• Calculation2 obj=new Calculation2();
• obj.sum(10.5,10.5);
• obj.sum(20,20);
•
• }
• }
• Test it NowOutput:21.0
Why Method Overloaing is not possible by changing
the return type of method?
• class Calculation3{
• int sum(int a,int b){System.out.println(a+b);}
• double sum(int a,int b){System.out.println(a+b);}
•
• public static void main(String args[]){
• Calculation3 obj=new Calculation3();
• int result=obj.sum(20,20); //Compile Time Error
•
• }
• }
• Test it Nowint result=obj.sum(20,20); //Here how can java
determine which sum() method should be called
• class Overloading1{
• public static void main(int a){
• System.out.println(a);
• }
•
• public static void main(String args[]){
• System.out.println("main() method invoked");
• main(10);
• }
• }
• Test it NowOutput:main() method invoked 10
Method Overriding in Java
• If subclass (child class) has the same method
as declared in the parent class, it is known
as method overriding in java.
• In other words, If subclass provides the
specific implementation of the method that
has been provided by one of its parent class, it
is known as method overriding.
Usage of Java Method Overriding
• Method overriding is used to provide specific
implementation of a method that is already
provided by its super class.
• Method overriding is used for runtime
polymorphism
Rules for Java Method Overridin
• method must have same name as in the
parent class
• method must have same parameter as in the
parent class.
• must be IS-A relationship (inheritance).
•
•
•
•
•
•
•
•
•
•
•
class Vehicle{
void run(){System.out.println("Vehicle is running");}
}
class Bike2 extends Vehicle{
void run(){System.out.println("Bike is running safely");}
public static void main(String args[]){
Bike2 obj = new Bike2();
obj.run();
}
Test it NowOutput:Bike is running safely
class Bank{
int getRateOfInterest(){return 0;}
}
class SBI extends Bank{
int getRateOfInterest(){return 8;}
}
class ICICI extends Bank{
int getRateOfInterest(){return 7;}
}
class AXIS extends Bank{
int getRateOfInterest(){return 9;}
}
class Test2{
public static void main(String args[]){
SBI s=new SBI();
ICICI i=new ICICI();
AXIS a=new AXIS();
System.out.println("SBI Rate of Interest: "+s.getRateOfInterest());
System.out.println("ICICI Rate of Interest: "+i.getRateOfInterest());
System.out.println("AXIS Rate of Interest: "+a.getRateOfInterest());
}
}
Test it NowOutput: SBI Rate of Interest: 8 ICICI Rate of Interest: 7 AXIS Rate of Interest: 9